In the related art, techniques for controlling operations of various safety devices according to a weight of a passenger sitting in a seat are known to improve performances of the safety devices equipped in a vehicle such as seat belts and airbags. For example, it is general that a warning light of “Fasten Seat Belt” is displayed after a detection of the sitting when the passenger sits in the seat but does not fasten the seat belt. In addition, according to regulations in practice in North America, the airbag has to be deployed in case of an accident in a case where an adult sits in an assistant driver's seat. In this manner, it is very important, in terms of safety, to detect and accurately determine the weight of the passenger.
As an example of the related art, JP 2003-83707A (Reference 1) discloses a strain sensor that includes a sensor substrate (strain body) which has two fixed holes fixed to respective members disposed in both end portions of a long plate, a detection hole arranged between the two fixed holes to be fixed to the other member, and at least one strain detection element adhered to an upper surface of the long plate between the fixed hole and the detection hole, the strain sensor further including a fixing member that is press-fitted into each of the fixed holes of the sensor substrate, and a detection member (connection member) that is press-fitted into the detection hole of the sensor substrate. An external force is applied to the end portion of the detection member abutting against the sensor substrate and a reaction force is applied to the end portion of the two fixing members abutting against the sensor substrate, and a strain is detected by using the strain detection element adhered to the sensor substrate based on the external force and the reaction force.
In the strain sensor according to JP 2003-83707A (Reference 1), a plurality of strain detection elements are arranged in parallel on a central straight line in a longitudinal direction of the sensor substrate (strain body). These strain detection elements can detect a load value or the like by using the strain caused by pulling and compression with respect to one direction (direction parallel with a gauge length). Accordingly, in a case where a load of the passenger in an up-down direction added to the vehicle seat is transmitted to the sensor substrate, the weight of the passenger can be detected by using the strain of the strain detection element arranged in the sensor substrate when the strain sensor according to JP 2003-83707A (Reference 1) is interposed between the vehicle seat and a floor side fixed member, the detection hole is connected to a seat frame (seat side fixing member), and the fixed hole is connected to the floor side fixed member.
However, in a case where the passenger sits in the seat, a hip point where the weight of the passenger is applied is separated from a position of the seat frame where the sensor substrate is arranged, and thus the seat frame that extends horizontally in a left-right direction of the vehicle is subjected to bending. Between the seat side fixing member (member of the seat frame extending in a front-back direction) corresponding to both of the end portions of the seat frame and the floor side fixed member, the bending results in a relative angular displacement for which the seat side fixing member that extends in the front-back direction with respect to the floor side fixed member is directed to collapse inside in a vehicle width direction. Accordingly, a load error directed for the collapse to a seat frame side (inside) in the left-right direction which is bent is generated in the sensor substrate that detects the load of the passenger as a load generated in the up-down direction. The load error directed for the collapse inside in the vehicle width direction caused by the seating load affects the strain detection element of the sensor substrate, and thus the weight of the passenger may not be accurately detected.
In addition, although being rigid rectangular bodies, the floor side fixed member where the sensor substrate is arranged and the seat frame tend to be elastically deformed (refer to FIG. 15) such that a front side and a back side of the vehicle floor are relatively twisted in opposite directions about a central axis OW extending in the left-right direction through a center of the seat frame in the front-back direction in a case where a biased external force is applied to the vehicle, through a sudden braking or the like, to deform the floor side fixed member and the seat frame in addition to the load of the passenger in the up-down direction. When a moment load is applied in the floor side fixed member and the seat frame in this manner to relatively twist the right side and the left side in opposite directions about the central axis OW extending in the left-right direction, the relative angular displacement may be generated about the central axis OW extending in the left-right direction between the floor side fixed member and the seat frame. The relative angular displacement affects the strain of the strain detection element that performs the weight detection, and thus the weight of the passenger may not be accurately detected.
Accordingly, in the related art, a weight detection sensor in which the strain detection element toward the sensor substrate is connected to a Wheatstone bridge circuit as a variable resistor and, as illustrated in FIG. 32, two strain detection element units SU having first to fourth strain detection elements SG1 to SG4 each are arranged in strain bodies 20 arranged at four sites FL, RL, FR, and RR in a front, back, left, and right of an upper rail of the one vehicle seat is used such that the effect of the load errors caused by the two types of the relative angular displacement can be cancelled by the first to fourth strain detection elements SG1 to SG4 of the eight strain detection element units SU and only the weight of the passenger can be accurately detected.
However, it is desired that the weight detection sensor be capable of having a reduced number of components and reduced manufacturing costs, in addition to accurately detecting the load of the passenger, without being subjected to the effect of the load error caused by the relative angular displacement between the floor side fixed member and the seat frame.